Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material, without the high cost and time that is required to actually produce printing plates and set up a high-speed, high-volume, printing press to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy the requirements of customers, resulting in a large loss of profits. By utilizing pre-press color proofing, time and money can be saved.
One such commercially available image processing apparatus, which is depicted in U.S. Pat. No. 5,268,708, is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring colorant from a sheet of donor material to thermal print media by applying a sufficient amount of thermal energy to the donor material to form an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, a lathe bed scanning subsystem (which includes a lathe bed scanning frame, a translation drive, a translation stage member, a print-head, and a vacuum imaging drum), and thermal print media and donor material exit transports.
The operation of the image processing apparatus of U.S. Pat. No. 5,268,708 comprises metering a length of the thermal print media (in roll form) from the material assembly or carousel. The thermal print media is then measured and cut into sheet form of the required length, transported to the vacuum imaging drum, registered, wrapped around and secured onto the vacuum imaging drum. Next a length of donor material (in roll form) is also metered out of the material supply assembly or carousel, measured and cut into sheet form of the required length. It is then transported to and wrapped around the vacuum imaging drum, such that it is superposed in the desired registration with respect to the thermal print media (which has already been secured to the vacuum imaging drum).
After the donor material is secured to the periphery of the vacuum imaging drum, the scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the thermal print media and the donor material on the spinning vacuum imaging drum while it is rotated past the print head that will expose the thermal print media. The translation drive then traverses the print head and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum. These movements combine to produce the intended image on the thermal print media.
The lathe bed scanning frame provides the structure to support the vacuum imaging drum and its rotational drive. The translation drive with the translation stage member and print head are supported by two translation bearing rods that are substantially straight along their longitudinal axis and are positioned parallel to the vacuum imaging drum and a lead screw. Consequently, they are parallel to each other therein forming a plane, along with the vacuum imaging drum and lead screw. The translation bearing rods are, in turn, supported by the outside walls of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine. The translation bearing rods are positioned and aligned there between, for permitting low friction movement of the translation stage member and the translation drive. The translation bearing rods are sufficiently rigid for this application, so as not to sag or distort between the mounting points at their ends. They are arranged to be as exactly parallel as is possible with the axis of the vacuum imaging drum. The front translation bearing rod is arranged to locate the axis of the print head precisely on the axis of the vacuum imaging drum with the axis of the print head located perpendicular, vertical, and horizontal to the axis of the vacuum imaging drum. The translation stage member front bearing is arranged to form an inverted "V" and provides only that constraint to the translation stage member. The translation stage member with the print head mounted on the translation stage member, is held in place by its own weight. The rear translation bearing rod locates the translation stage member with respect to rotation of the translation stage member about the axis of the front translation bearing rod.
In U.S. Pat. No. 5,268,708, the translation stage member and print head are attached to a rotatable lead screw (having a threaded shaft) by a drive nut and coupling. The coupling is arranged to accommodate misalignment of the drive nut and lead screw so that only rotational forces and forces parallel to the lead screw are imparted to the translation stage member by the lead screw and drive nut. The lead screw rests between two sides of a lathe bed scanning frame of the lathe bed scanning subsystem or write engine, where it is supported by deep groove radial bearings. At the drive end the lead screw continues through the deep groove radial bearing, through a pair of spring retainers, that are separated and loaded by a compression spring to provide axial loading, and to a DC servo drive motor and encoder. The DC servo drive motor induces rotation to the lead screw moving the translation stage member and print head along the threaded shaft as the lead screw is rotated. The lateral directional movement of the print head is controlled by switching the direction of rotation of the DC servo drive motor and thus the lead screw.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. In order to achieve the positioning accuracy for high-resolution imaging at 1800 dots per inch or greater, the apparatus described above utilizes a lead screw having a very fine thread pitch. Approaches to this problem disclosed in co-pending application Ser. No. 09/144,390 filed on Aug. 31, 1998 allow a coarser lead screw pitch to be used.
It can be appreciated that a significant amount of design work is required to maintain synchronization and dot addressability in an imaging apparatus where a print head, possibly having a variable number of light sources, is moving linearly along a high-speed rotating imaging drum. To achieve the necessary timing for this imaging task, a specific lead screw thread pitch is selected for the imaging resolution that is required. Co-pending application Ser. No. 09/144,390 filed on Aug. 31, 1998 discloses a method and example for calculating lead screw pitch for an apparatus imaging at 2540 dots per inch.
It would be advantageous to be able to readily change the resolution of an imaging apparatus to suit different requirements of end-customers who use such equipment. For example, there are significant advantages for an image processing apparatus that could operate at both 2540 dots per inch and at 2400 dots per inch. A preferred solution for meeting this requirement is to enable each resolution using a different lead screw pitch.
It will be appreciated that changing the lead screw in a high-resolution imaging apparatus presents considerable problems. Conventional solutions would require a significant amount of disassembly to loosen the lead screw from mounting, fastening, and support hardware at each end and to install the alternate lead screw in its place. Service costs for lead screw replacement at an end-customer site would limit the market value of such a solution. End-customers would be likely to reject conventional solutions for lead screw replacement as troublesome, costly, time-consuming, and error-prone.
Lead screw replacement conventionally requires tools and involves well-trained personnel to make necessary adjustments so that synchronization timing can be maintained. Patents that disclose methods for lead screw replacement include U.S. Pat. No. 4,628,171, which discloses a mechanical-feed boring machine tool with interchangeable lead screws, where different lead screw pitches are needed to change the threading pitch achieved by this machine. Conventional hand tools and detailed disassembly procedures are required to substitute another lead screw having a different thread pitch with this approach.
The apparatus disclosed in U.S. Pat. No. 5,771,059 employs a magnet integrally attached to the lead screw that allows one end of the lead screw to be removed from its position in the scanning frame. Also, the apparatus disclosed in co-pending application Ser. No. 08/795,171 uses a magnetically loaded radial bearing integrated with the lead screw shaft that allows the opposite end of the lead screw to be securely held in position within a frame, while at the same time providing a bearing to allow rotational movement. However, none of the arrangements noted above show or suggest a structure or method which permits the removal and the re-seating of a complete lead-screw assembly without requiring tools.